1. Field of the Invention
The present invention relates to a liquid droplet discharge head and an image forming apparatus, and more specifically, to a liquid droplet discharge head and an image forming apparatus in which nozzles which discharge liquid droplets are arranged in a two-dimensional matrix array.
2. Description of the Related Art
Inkjet recording apparatuses (inkjet printers) having an inkjet head (ink ejection head) in which a plurality of nozzles are arranged, are known as image forming apparatuses. An inkjet recording apparatus of this kind forms images by forming dots on a recording medium, by ejecting ink as droplets from nozzles, while causing the inkjet head and the recording medium to move relatively to each other.
Various methods are known conventionally as ink discharge methods for an inkjet recording apparatus of this kind. For example, one known method is a piezoelectric method, where the volume of a pressure chamber (ink chamber) is changed by causing a diaphragm forming a portion of the pressure chamber to deform due to deformation of a piezoelectric element (piezoelectric actuator), ink being introduced into the pressure chamber from an ink supply passage when the volume is increased, and the ink inside the pressure chamber being ejected as a droplet from the nozzle when the volume of the pressure chamber is reduced. Another known method is a thermal inkjet method where ink is heated to generate a bubble in the ink, and ink is then ejected by means of the expansive energy created as the bubble grows.
In an inkjet recording apparatus, one image is represented by combining dots formed by ink ejected from the nozzles. High image quality can be achieved by making the dots small in size, increasing the density of the dots and by using a large number of pixels per image.
FIG. 19, for example, shows an enlarged view of a portion of an inkjet head in which nozzles are arranged in a two-dimensional matrix array. The inkjet head 90 shown in FIG. 19 records images by discharging ink from the nozzles 91 (91a, 91b, 91c), onto a recording medium (not illustrated) which is conveyed relatively to the inkjet head 90. The inkjet head 90 is disposed in such a manner that the lengthwise direction of the head is aligned with the breadthways direction of the recording medium (a main scanning direction), which is perpendicular to the direction of conveyance of the recording medium (the sub-scanning direction).
Pressure chambers 92 correspond respectively to each nozzle 91 of the inkjet head 90. As shown in FIG. 19, the nozzles 91 are disposed respectively in the main scanning direction and the sub-scanning direction, thereby forming a two-dimensional matrix arrangement. In this case, the direction in which the nozzles 91 are arranged in the sub-scanning direction does not coincide totally with the sub-scanning direction (the direction perpendicular to the main scanning direction), but rather, they are arranged at a slightly oblique angle with respect to the sub-scanning direction. For example, the distance, Pm, in the main scanning direction between nozzles which are mutually adjacent in the sub-scanning direction, such as nozzle 91a and nozzle 91b is clearly smaller than the distance L1 between the nozzle 91a and the nozzle 91c adjacent to same in the main scanning direction (this distance being equal to the approximate size of a pressure chamber 92).
By arranging the nozzles 91 at a slight oblique angle with respect to the sub-scanning direction in this way, after a dot 93a has been formed by discharging ink onto the recording medium from the nozzle 91a, for example, the recording medium is conveyed through a distance corresponding to the size L2 of a pressure chamber 92, in the sub-scanning direction, and if ink is then discharged onto the recording medium from nozzle 91b, it will form a dot 93b that is directly alongside the dot 93a formed previously by nozzle 91a, in the main scanning direction. The distance between the centers of these dots (the center-to-center distance) is equal to the distance, Pm, in the main scanning direction between the nozzles (91a and 91b) which are mutually adjacent in the sub-scanning direction as described above. In this way, by arranging nozzles 91 in a matrix fashion, and positioning this matrix at a slight oblique angle, it is possible to achieve high density of the nozzles (which means a high density of the dots formed by these nozzles).
For example, Japanese Patent Application Publication No. 9-507803 describes an inkjet head in which nozzles are arranged in a two-dimensional matrix array comprising n rows and m columns, in such a manner that the connections to the respective individual electrodes are reduced and high density is achieved.
Furthermore, a line type inkjet head is known in which respective head chips having a plurality of ink nozzles arranged in a single row are arrayed on the same substrate in a staggered two-row fashion, at an oblique angle with respect to the direction of arrangement (see Japanese Patent Application Publication No. 2002-273878, for example).
However, in high-speed inkjet head printing using a line head in which the nozzles are arranged at high density, since the droplet ejection intervals between respective liquid droplets is very short, a phenomenon known as “landing interference” or “droplet ejection interference” may occur, in which the liquid droplets discharged onto the recording medium make contact and overlap with each other before becoming fixed in the recording medium, the droplets combining to form one big droplet, or the shapes of the dots becoming disrupted as they permeate into the recording medium, thus leading to bleeding, color mixing, and the like. This causes image quality to decline. The coalescence of the liquid droplets occurs not only in the sub-scanning direction, which is the conveyance direction of the recording medium, but also in the main scanning direction perpendicular to the sub-scanning direction. If coalescence of liquid droplets occurs in two dimensions in this way, then particularly significant image degradation occurs.
Moreover, in a conventional inkjet head as illustrated in FIG. 19, since the nozzles are simply arranged at an oblique angle to the sub-scanning direction, then after respective ink droplets which are mutually adjacent in the main scanning direction have landed on the recording medium, the droplets coalesce before becoming fixed and hence form a large droplet. This leads to image degradation.
Moreover, in the device disclosed in Japanese Patent Application Publication No. 9-507803, the nozzles are simply arrayed in a two-dimensional matrix arrangement, and there is no particular disclosure regarding the method of arranging the nozzles. Therefore, it involves problems similar to those of conventional inkjet heads as described above.
Moreover, the device disclosed in Japanese Patent Application Publication No. 2002-273878 has the objective of achieving high density in a line type head, and it does not disclose the relationship between the dot diameter and nozzle arrangement, in order to prevent landing interference. Therefore, if printing is carried out using a line head having the nozzle arrangement described in Japanese Patent Application Publication No. 2002-273878, then similarly to a conventional simple matrix head as illustrated in FIG. 19, there is a risk that dots which are discharged by adjacent nozzles in the main scanning direction will coalesce and aggregate before becoming fixed on the recording medium, and hence degradation of image quality will occur.